The invention relates to an internal combustion engine such as the drive motor of a portable handheld work apparatus including a motor-driven chain saw, a brushcutter, a cutoff machine or the like.
In known two-stroke engines, a choke flap is mounted in the carburetor upstream of the throttle flap. The choke flap is closed except for a start cross section in the case of starting for enriching the mixture. In the start case, it should be simultaneously ensured that the throttle element mounted in the air channel closes completely so that no air can flow into the transfer channels via the air channels in the start case. This would lead to a leaning of the mixture and this makes the start of the engine more difficult and hinders a problem-free runup of the engine. For this reason, the throttle in the air channel must be set to a start position with a position-dependent coupling to the choke flap.
It is an object of the invention to improve an internal combustion engine of the kind referred to above so that well defined operating conditions are provided without leaning the mixture for part-air positions especially when starting the engine.
The internal combustion engine of the invention includes an engine in a portable handheld work apparatus. The internal combustion engine includes: a cylinder having a cylinder wall; a piston mounted in the cylinder to undergo a reciprocating movement along a stroke path between top dead center and bottom dead center during operation of the engine; the cylinder and the piston conjointly delimiting a combustion chamber; a crankcase connected to the cylinder; a crankshaft rotatably mounted in the crankcase; a connecting rod connecting the piston to the crankshaft to permit the piston to drive the crankshaft as the piston reciprocates in the cylinder; at least one transfer channel connecting the crankcase to the combustion chamber; the transfer channel having a first end defining an entry window opening into the combustion chamber and a second end opening into the crankcase; an air channel connected to the transfer channel for supplying an essentially fuel-free gas flow thereto; a carburetor for supplying an air/fuel mixture; the carburetor having an intake channel and a carburetor throttle flap; an inlet channel downstream of the carburetor flap for conducting the air/fuel mixture into the crankcase; an air filter having an air filter housing; the air filter housing having a base wall and the base wall having an intake opening formed therein; the intake channel being connected to the intake opening upstream of the carburetor throttle flap; the base wall also having a bypass opening formed therein and the air channel being connected to the bypass opening; a common choke element provided for the intake opening and the bypass opening; and, the choke element being displaceable between an operating position wherein the intake opening and the bypass opening are clear and a start position wherein the cross section of the intake opening is reduced to a start cross section and the bypass opening is essentially closed.
The arrangement of a common choke element for the intake opening of the carburetor as well as for the bypass opening of the air channel makes possible a simple switchover from the start position into the operating position. The bypass opening can be closed separately from the carburetor intake opening. If the choke flap is pivoted back into the operating position after starting, the intake opening and the bypass opening are preferably enabled sequentially so that a switchover of the engine from the start state into the operating state is provided without the problem of the engine stalling. According to the invention, the choke element is assigned to the bypass opening or intake opening and need not lie in the channel connected to the opening so that a closing of the bypass opening or intake opening is possible in a simple manner by overlapping. Accordingly, there is no need to maintain tight tolerances. The choke element can be pushed transversely to the channel as a blocking slider.
The choke element is advantageously arranged as an external component in the clean air space of the air filter and is there provided as an approximately flat slider approximately parallel to the base of the housing. A slider can be moved in a simple manner without complex mechanics and is especially pivotable about a rotational axis so that the constructive complexity for the configuration of the choke element and its actuation is minor.
The choke element can be made of plastic in a simple manner and especially be configured as a plastic injection-molded part and so becomes a series part which can be manufactured in mass production. This part nonetheless prevents the penetration of unwanted air into the bypass opening or intake opening. With an advantageous configuration as a flat slider parallel to the housing base of the air filter, an underpressure, which builds up in the intake channel of the carburetor or in the throttle channel of the air channel, pulls the slider onto the intake opening to thereby close this opening tightly. This is especially significant for the throttle channel of the bypass air. With the first ignitions, the engine runs up to idle rpm and a corresponding underpressure is built up in the air channel which could facilitate the inflow of unwanted air and therefore cause a leaning of the mixture. Because of the configuration of the choke element of the invention as a flat slider, the underpressure which builds up leads to the situation that the plate of the slider, which closes the bypass opening, is drawn by suction and lays seal-tight on the edge of the bypass opening. In this way, a penetration of unwanted air into the air channel is reliably prevented just at the time point of the runup of the engine so that a leaning of the mixture during runup of the two-stroke engine is avoided.